Add new observation type: time bound

[jamessynge]

I'd like to be able to specify that a specific target is to be observed for a certain period of time (e.g. starting an hour before the start of a known transit, continuing until an hour hour after it finishes the transit).

[wtgee]

Yes, we've wanted a Time Constraint for a while. Note that much of the work has been done here: https://astroplan.readthedocs.io/en/latest/tutorials/periodic.html (although I've not looked into that since it's been up and we implement our constraints slightly different)

[wtgee]

See https://astroplan.readthedocs.io/en/latest/tutorials/periodic.html